There are two places you should check: one for Processor and one for iSeries.
Go to FFXMAIN, Option 5 (Configuration menu), Option 1 (System defaults).
1. Processor purge. Put a 2 on General System defaults, page down 1 screen, purge settings will be set at the top of the screen for eligible Outbound faxes and Outbound details. Values are Daily, Weekly and Monthly.
2. iSeries purge. Put a 2 on iseries environment, page down 1 screen, purge settings are set on the bottom of screen. Auto purge eligible data/files/memos. Values are Daily, Weekly and Monthly.